Electrodeposition technology is well known in the art for applying coatings, such as paint, to electrically conductive substrates.
Cathodic electrodeposition, wherein a positively charged coating dispersion is deposited on a negatively charged substrate, is generally preferred over anodic electrodeposition because cathodic electrodeposition exhibits better throw power in coating complex shapes. "Throw power" is the ability of an electrodeposition bath to deposit a uniform coating and cover recessed areas, such as inner surfaces of automobile door and hood panels, where conventional spray coating techniques cannot reach. Thus, the metal part coated by a cathodic electrodeposition (i.e., electrocoating) process generally exhibits improved corrosion resistance compared to similar parts coated by an anodic electrodeposition process. An excellent description of the cathodic electrodeposition process is provided by John Vincent in Paper 418, Automotive Cathodic Electrodeposition, Corrosion 91, available from the National Association of Corrosion Engineers (NACE).
A typical cathodic electrodeposition coating bath contains an epoxy-amine adduct or resin and a blocked polyisocyanate cross-linking agent. This mixture is partially neutralized with an organic acid and emulsified in water to form a resin feed package. A blend of pigments, selected to provide desired appearance, color, and other properties, is dispersed in a grinding medium as a pigment feed package. The two packages are continuously fed to the continuous electrodeposition bath to replenish the bath solids (non-volatiles) as metal parts are coated. Representative coating baths and cathodic electrodeposition processes are disclosed in U.S. Pat. No. 5,356,960 to Chung and Gam; U.S. Pat. No. 3,922,253 to Jarabek et al.; U.S. Pat. No. 4,419,467 and U.S. Pat. No. 4,468,307 to Cuismer et al; and U.S. Pat. No. 4,137,140 to Belanger.
Electrocoated metals may be used in applications where they are exposed to friction with other metal parts, such as seat tracks in vehicles, or to friction with materials such as engineering plastics (e.g., plastic bushings in vehicle window tracks). The cured electrodeposited coating is not self lubricating, and thus typically is lubricated with a surface coating of dry lubricant or grease for such applications where friction is encountered.
Dry fluorinated resin lubricants such as poly(tetrafluoroethylene), lubricate well under light loads, but typically do not perform well under heavy loads. Moreover, such lubricants require separate application and are easily removed in use. Greases also readily transfer to adjacent surfaces (e.g., carpet or glass), or by accidental contact with the skin or clothes of those using the electrocoated metal parts. Greases also may require periodic reapplication, and may degrade properties of engineering plastics, such as those commonly used in plastic bearings. Thus, there is an ongoing need to improve the self-lubrication properties of cathodic electrodeposition coatings.
Highly fluorinated polymers, such as poly(tetrafluoroethylene), are well known in the art as having self-lubricating (i.e., "slippery") surfaces. It has been proposed to add such fluorinated polymers to various compositions to improve lubricity (i.e., self-lubrication) to the composition. For example, Hosoda et al., disclose a primer composition containing an epoxy resin, an aromatic polyamine, a polyisocyanate and a lubricant, which may be a fluorinated resin, to improve press formability, in a solvent such as cyclohexanone (U.S. Pat. No. 5,468,461).
Fluorinated resins have not heretofore been incorporated in cathodic electrodeposition coating baths since such resins are nonionic, and thus will not migrate to the cathode during the electrocoating process.